The present invention relates to a braking apparatus for vehicles, which is equipped with a brake fluid pressure retaining means for continuously retaining brake fluid pressure after a release of a brake pedal.
Various braking apparatus for vehicles are known, which are equipped with a braking force retaining means or a brake fluid pressure retaining means for continuously acting a braking force after a release of a brake pedal. For example, the applicants disclose a brake fluid pressure retaining unit or a braking apparatus for vehicles in Japanese Patent Application No. Hei-10-370249 (unpublished), in which two systems of brake fluid pressure passages are provided, and each of the brake fluid pressure passages is equipped with a solenoid valve and a brake fluid pressure reduction speed control means for retaining reduction speed of the brake fluid pressure within a wheel cylinder less than that of the brake pedal load applied by a driver. In this braking apparatus, the brake fluid pressure is retained within the wheel cylinder after a release of the brake pedal for continuously acting a braking force, and thereby a smooth and reliable starting operation on a slope can be achieved without undesirably rolling down the slope.
However, in the prior art braking apparatus, the retained brake fluid pressure is released at a time, and so the braking force is immediately reduced to zero at the time of the release, which leads to a sudden start of the vehicle. The driver feels this sudden feel as a sudden impact or brake dragging. This is more prominent on a down slope because the vehicle""s own weight also affects the driving force of the vehicle. Further, if the braking force is immediately reduced to zero at the release of the brake, slippage or idle spin of the driving wheels occurs upon starting the vehicle on a slippery road with a small coefficient of frictional resistance (xcexc), such as a snowy road or icy road, due to the relation between the braking force and the driving force. Therefore, the driver cannot perform a smooth start of the vehicle.
In view of the above, the main object of the present invention is to provide a braking apparatus for vehicles, which ensures a smooth start of the vehicle without any sudden feels at the start.
In order to solve the above objective, the inventors made considerable research and development works and finally completed the present invention in consideration of the fact that the brake fluid pressure passage of the vehicle is separated into a plurality of individual passages and that in the conventional braking apparatus the retained brake fluid pressure is released simultaneously in all wheel cylinders. According to the present invention, there is provided a braking apparatus for vehicles, in which plurality systems of individually separated brake fluid pressure passages are arranged, and each passage is provided with at least one brake fluid pressure retaining means for continuously retaining brake fluid pressure within a wheel cylinder arranged in the brake fluid pressure passage after a depression of a brake pedal is released and for releasing the retained brake fluid pressure in response to an increase of a starting driving force of the vehicle, wherein the release of the retained brake fluid pressure is carried out in a time differential manner at each brake fluid pressure retaining means.
In such a construction, even if a depression of the brake pedal is released, the brake fluid pressure retaining means continuously retains the brake fluid pressure within the wheel cylinder. As a result, when the vehicle stops on a slope, the driver can start the vehicle without undesirable backward displacement of the vehicle. The retained brake fluid pressure is released in response to an increase of the starting driving force.
Further, in this braking apparatus for vehicles, the brake fluid pressure passage is separated into plurality systems, and each wheel is equipped with a wheel cylinder. Therefore, brake fluid pressure retaining means may be provided in each system of the plurality brake fluid pressure passages. Alternatively, the brake fluid pressure retaining means may be provided at each wheel cylinder. Additionally, the brake fluid pressure retained by these brake fluid pressure retaining means is released in a time differential manner. That is, in response to an increase of the starting driving force of the vehicle, the retained brake fluid pressure is released at each system of the brake fluid pressure passages or at each wheel cylinder with a time lag between one system and the others or one wheel cylinder and the others. For this reason, the retained brake fluid pressure is not released at one time, and so a release of the braking force is smoothly carried out.
In a four-wheel vehicle where the brake fluid pressure passage is separated into two individual systems and each brake fluid pressure passage of the system is further divided and connected to two wheel cylinders, brake fluid pressure retaining means are preferably provided in the following manner. This is because the release of the brake fluid pressure can be performed with a time lag and redundant arrangement of equipment can be eliminated.
(a) One brake fluid pressure retaining means is provided at each brake fluid pressure passage of the two systems, preferably at a point before the brake fluid pressure passage is divided.
(b) One brake fluid pressure retaining means is provided at respective wheel cylinders, and at least two wheel cylinders out of four wheel cylinders are equipped with the brake fluid pressure retaining means. The brake fluid pressure retaining means is provided in the brake fluid pressure passage at a point after the brake fluid pressure passage is divided, viz. close to the wheel cylinder.
(c) In the case that a brake fluid pressure retaining means is provided in one system of the brake fluid pressure passage out of two systems at a point before the brake fluid pressure passage is divided, a brake fluid pressure retaining means is provided in the other system of the brake fluid pressure passage at least at a point after the brake fluid pressure passage is divided, viz. close to the wheel cylinder.
Herein, the wording xe2x80x9can increase of a starting driving force of the vehiclexe2x80x9d includes, but is not limited to, a case where no starting driving force has arisen, a case where a sufficient starting driving force has not arisen to prevent the vehicle from undesirably rolling down a slope, and a case where a starting driving force has increased to such an extent that the vehicle can start on a slope without undesirable backward displacement. The increase of the starting driving force may be determined by directly measuring a driving torque of the driving wheel, however, it can also be determined based on the following points:
(a) a point when the driver depresses the accelerator pedal, in case of a vehicle with an automatic transmission;
(b) a point when the driver depresses the accelerator pedal and connects the clutch, in case of a vehicle with a manual transmission; and
(c) in case of a vehicle with an automatic transmission which automatically increases a driving force transmission capacity of the starting clutch, in response to a release of the brake pedal, to such an extent that the driving force of the vehicle increases until the vehicle withstands an undesirable backward displacement on a slope, a point when such an increase is achieved, for example a point when a creep driving force has arisen or a point when a strong creep condition is achieved as stated in the detailed description of the preferred embodiments.
Further, the wording xe2x80x9cthe release of the retained brake fluid pressure is carried out in a time differential mannerxe2x80x9d includes, but is not limited to, the following cases:
(a) a starting point of the release of the retained brake fluid pressure is different at each brake fluid pressure retaining means (the release of the brake fluid pressure may be completed at the same time with other brake fluid pressure retaining means); and
(b) a starting point of the release of the retained brake fluid pressure is common in all the brake fluid pressure retaining means, and a finishing point of the release of the brake fluid pressure is different at each brake fluid pressure retaining means (for example, a release speed of the brake fluid pressure differs at each brake fluid pressure retaining means).